Obviously, there is always a cause. Finding it is the problem. I would suggest looking for any missing fasteners in the area of the noise. Pressing down on various assemblies with the engine running is helpful too, with the usual warnings about moving parts, and losing fingers. It is best to start diagnosing with a cold engine, since so many parts become too hot to touch after a short time. In the end, like the OP, many just give up, and chalk it up to owning an aging car.